3D resonator filters such as cavity combline, dielectric resonator and waveguide filters are widely used in wireless communication applications due to their superior performance in terms of high quality factor (Q-values) and high power handling capability. Several frequency bands are utilized simultaneously in wireless base stations to support different wireless standards. Each frequency band requires the use of bandpass filters to suppress unwanted signals and avoid the interference from adjacent bands. Using the conventional method, several bandpass filters are required to be installed in a base station to meet such requirements. Moreover, any upgrade of the network to accommodate a new standard, will require the addition of new filters to the base station. The availability of tunable/reconfigurable hardware helps to reduce the base station size by reducing the number of filter elements, it also provides the network operator the means for efficiently managing hardware resources, while accommodating multi-standards requirements and achieving network traffic/capacity optimization. Tunable filter also allows a base station to be upgraded for future wireless standards without any need for installation of new filters.
In order to minimize the number of tuning elements and to improve the loss performance of the tunable filter, it is preferable to use tuning elements only to tune the resonator center frequencies. However, the variation of inter-resonator coupling with frequency is different from that of the input/output coupling. This in turn results in deterioration of the filter return loss and changes in the filter absolute bandwidth over the tuning range. One possible solution is to add tuning elements to control the inter resonator coupling and the input/output coupling as well. In many cases, this solution may not be even feasible because of size limitation, design complexity and the inherent difficulty to tune sequential and cross inter-resonator coupling. Therefore, one needs to use only tuning elements for the resonators to tune their frequency.
This invention discloses a design method and structure of a 3D tunable bandpass filter, which avoids complex structures and provides a constant absolute bandwidth with thorough use of tuning elements only for the resonators.